The present invention relates to a temperature sensor, in particular for a gas sensor.
Temperature sensors are known. They are used in gas sensors to detect and regulate an operating temperature of the gas sensor. Such gas sensors are needed, for example, to determine the oxygen partial pressure in the exhaust gas from an internal combustion engine. They are usually constructed in layers, with the individual layers containing the elements needed to ensure functionality.
The individual layers are made of a ceramic material which is permanently applied by film casting, punching, screen printing, laminating, cutting, sintering or the like. Individual ceramic layers have a printed conductor made of a cermet or they are made of a solid electrolyte and thus have the ability to conduct electricity. Such gas sensors have at least one area where a solid electrolyte is applied between at least two cermet electrodes. The conductivity of the solid electrolyte varies as a function of the oxygen partial pressure, for example, and thus is a direct measure of the oxygen concentration in the exhaust gas. However, for an exact determination of the measured value, the temperature must be above 300xc2x0 C. because the solid electrolyte has the required basic conductivity only above such a temperature, and gas sensors therefore have at least one heating device.
On the other hand, the temperature of the exhaust gas varies greatly during operation of an internal combustion engine. Since the conductivity of the solid electrolyte depends on temperature, this change in temperature must be determined.
It is known that such gas sensors may be equipped with temperature sensors to detect and analyze a temperature-dependent change in resistance of a printed conductor.
First, the resistance of the printed conductor of the heating device may be measured. Printed conductors are usually made of a cermet, using platinum, for example, as the metal. The resistance is then dominated by the metallic conducting platinum. One disadvantage of this is that the resistance temperature coefficient is low, and this low dependence of resistance on temperature is often inadequate to construct a suitable control device.
In addition, temperature can be detected on the basis of the resistance of the solid electrolyte, in which case the solid electrolyte with the known sensors has a double function. The solid electrolyte has a much higher resistance temperature coefficient. A disadvantage of the known methods is that the resistance also depends on various external factors such as the oxygen partial pressure, activation, aging and poisoning, so that accurate determination of the measured value is no longer possible.
The temperature sensor according to the present invention offers the advantage that it permits much more accurate determination of measured values. Due to the fact that the printed conductor has at least one section made of a solid electrolyte, the favorable resistance temperature coefficient can be utilized while, on the other hand, this section is not exposed to the various interfering external factors such as poisoning, activation, aging and oxygen partial pressure.
It is especially advantageous to cover the printed conductor of the temperature sensor with an insulation layer. Such an insulation layer may be made of aluminum oxide, for example.
It is also advantageous to integrate the temperature sensor into a layer carrying the heating device, because it is delineated from the other elements of the gas sensor by insulation layers. In this case, to simplify the production of the gas sensor, a partial section of the printed conductor of the heating device may correspond to a partial section of the printed conductor of the temperature sensor. The section of the temperature sensor having the solid electrolyte is in contact with the printed conductor of the heating device and is also in contact with a printed conductor of the temperature sensor.
It is also preferable for the temperature-dependent change in resistance to be detected by an alternating voltage acting on it.